1. Field of the Invention
The present invention relates to an apparatus for detecting the level of a liquid specimen, for example, in a test tube or tubes in an automatic analyzer.
2. Description of the Related Art
In an automatic analyzer, a vertically movable mechanism is used for a nozzle for taking up a liquid specimen, such as a blood plasma and corpuscle.
In the case where such a nozzle is employed, it is necessary to detect the contact of the nozzle with the liquid specimen. Unless the contact level can be detected, there is a risk that the nozzle will be immersed too deep or that the descent motion of the nozzle will be mechanically continued on even if a test tube becomes empty. This may cause an adverse effect on the result of analysis of the liquid specimen.
A conventional technique has been developed for detecting the level of a liquid specimen. It is known to detect the level of a liquid through its electrical conductivity. In this method, a pair of electrodes are inserted into a test tube in interlock with an associated nozzle and hence moved vertically down relative to the surface level of the liquid specimen. When the pair of electrodes are brought down to the liquid level, an electric current is carried from one electrode to the other electrode through the conduction of the liquid specimen. It is, therefore, possible to detect the surface level of the liquid specimen by detecting the electrical conduction.
In this method, the electrodes can be mounted integral with a nozzle for liquid level detection, as disclosed in U.S. Pat. No. 4,939,925 and Published Unexamined Japanese Patent Application 61-117460.
Since the test tube is normally narrow in diameter, the nozzle, unless being small-sized, cannot be inserted into the test tube. In particular, a nozzle of synthetic resin is relatively large and improper to insert normally into the test tube.
Forming the electrodes integral with the nozzle can somewhat decrease their occupation area, but the nozzle by itself becomes expensive and is not suitable as a dispensable one.
It is known, as in U.S. Pat. No. 3,635,094, to detect the immersed state of a nozzle in a liquid specimen through the utilization of an electrostatic capacitance involved.
In this method, a test tube or both a support stand and nozzle are made of a conductive material and serves as a pair of electrodes. When the nozzle is immersed in a liquid level, it serves as one electrode against the test tube acting as the other electrode, thus leading to a variation in electrostatic capacitance between the two. The immersion of the nozzle in the liquid specimen can be detected based on a variation in their electrostatic capacitance.
In this detection method, however, there is involved no variation in electrostatic capacitance, unless the nozzle is immersed in the liquid nozzle. In this case it is possible to only detect the fact that the nozzle has been immersed into the liquid specimen. Stated in a stricter sense, this cannot be regarded as a proper detection of the liquid level, offering a detection accuracy problem. Further, the test tube or support stand is restricted principally to a material of high conductivity because the purpose of the material is to enable the test tube or support stand to act as an electrode. Further, the potential on the support stand is held constant so as to provide a reference against the electrode. For this reason, it is not possible to use any inexpensive synthetic resin and glass, resulting in high cost. In the case where the test tube is shifted relative to the support stand, attention should be paid to any defective electrical contact. Thus the positive detection of the liquid level is, therefore, affected.